Heat exchanger and distillation column arrangement

ABSTRACT

A shell and tube heat exchanger and distillation column arrangement for an air separation plant utilizing such heat exchanger in which tubes for passage of a liquid that is used in condensing a vapor are located within a cylindrical shell. The tubes are arranged in an inner array of tubes and an outer array of tubes surrounding the inner array of tubes and having more tubes than the inner array. The inner array of tubes present a larger average area, between tubes, for flow of the vapor in an outward, radial direction than tubes of the outer array to lessen pressure drop while allowing for more tubes to be located within the shell to increase the surface area available for heat exchange between the liquid and the vapor.

FIELD OF THE INVENTION

The present invention provides a shell and tube heat exchanger havingtubes located within a shell and a distillation column arrangementuseful for air separation in which the heat exchanger operablyassociates higher and lower pressure columns in a heat transferrelationship to vaporize an oxygen-rich liquid produced in the lowerpressure column through indirect heat exchange with a nitrogen-richvapor produced in the higher pressure column to condense thenitrogen-rich vapor. More particularly, the present invention relates tosuch a heat exchanger in which the tubes are arranged in inner and outerarrays and the inner arrays present a greater average open area betweenthe tubes thereof than that of the outer array of tubes to lowerpressure drop.

BACKGROUND OF THE INVENTION

Shell and tube heat exchangers are used in a variety of industrialprocesses to indirectly exchange heat between a liquid and a vapor. Suchheat exchangers have an arrangement of tubes located within a shell. Theliquid is introduced into the tubes where the liquid at least partiallyvaporizes through indirect heat exchange with a vapor that is introducedinto the shell. As a result of the heat exchange, the vapor condensesand the condensate is discharged from the shell. Typically, the tubesare supported by tube sheets located at opposite ends of the shell thatis of cylindrical configuration. One of the tube sheets has an inlet forthe vapor and the other of the tubesheets has an outlet for dischargingthe resulting condensate.

Shell and tube heat exchangers are used in a variety of locations withinan air separation plant. In an air separation plant, air is compressedand then purified of higher boiling impurities such as water vapor andcarbon dioxide in a pre-purification unit. The pre-purification unitemploys beds of adsorbent that are operated in an out-of-phase cycle toadsorb the water vapor and carbon dioxide and thus, produce a compressedand purified air stream. The pre-purification unit can also be designedto remove carbon monoxide and hydrocarbons that may be present in theair. The resulting compressed and purified air is then cooled to atemperature that is at or near the dew point of the air and thenintroduced into a distillation column arrangement having higher andlower pressure columns that are arranged in a heat transfer relationshipby a shell and tube heat exchanger such as described above. In thisregard, the higher pressure column will typically operate at between 5and 6 bara and the lower pressure column will operate at between 1.1 and1.5 bara.

The air is separated within the higher pressure column into anitrogen-rich vapor column overhead and a crude liquid oxygen columnbottoms also known as kettle liquid. A stream of the kettle liquid isfurther refined in the lower pressure column into a nitrogen-rich vaporcolumn overhead and an oxygen-rich liquid. In an air separation plantthe shell and tube heat exchanger is employed as a condenser-reboiler tocondense a stream of the nitrogen-rich vapor through indirect heatexchange with the oxygen-rich liquid, thereby to partially vaporize theoxygen-rich liquid and provide boilup to the lower pressure column. Sucha condenser reboiler can be situated within a sump of the lower pressurecolumn. The liquid nitrogen is used both as reflux to the higher andlower pressure columns and also, optionally, as a liquid product. It isto be noted that shell and tube heat exchangers are used in other placeswithin the air separation plant, for instance, as an argon condenser tocondense argon within an argon column attached to the lower pressurecolumn to produce an argon product.

U.S. Pat. No. 4,436,146 illustrates a shell and tube heat exchangeremployed as a condenser reboiler in a double column arrangement of anair separation plant. The condenser reboiler functions to condensenitrogen-rich vapor overhead of the higher pressure distillation columnthrough indirect heat exchange with an oxygen-rich liquid produced inthe lower pressure column. The resulting nitrogen-rich vapor willcondense at the higher pressure of the higher pressure column andthereby supply the heat of vaporization for the oxygen-rich liquid. Inthe shell and tube heat exchanger shown in this patent, the tubes areopen at opposite ends that the heat exchanger sits in a pool of theoxygen-rich liquid that collects as a column bottoms within the lowpressure column. The vaporization of the oxygen-rich liquid within thetubes entrains the liquid which rises in the tubes. The resultingoxygen-rich vapor provides boilup within the lower pressure column andunvaporized or residual oxygen-rich liquid is returned to the pool ofthe oxygen-rich liquid collected in the bottom of the lower pressurecolumn. Such a heat exchanger is known as a thermosiphon reboiler. Thenitrogen-rich vapor condenses on the exterior of the tubes and isdischarged to a piping network from which the resulting nitrogen-richliquid is introduced into both the higher and lower pressure columns asreflux and can be taken as a liquid product.

U.S. Pat. No. 4,436,146 incorporates features that are employed incondenser reboilers and also, shell and tube heat exchangers generally.For instance, a baffle plate is provided opposite to the vapor inlet tohelp urge the flow of the incoming nitrogen vapor in an outward, radialdirection of the shell. Beneath this plate is an elongated cylindricalmember that will collect non-condensable substances in the air such asneon and helium. Additionally, the area provided at the exterior of thetubes for heat transfer can be enhanced by provision of length-wiseextending fins, also known as fluting. Further, the shell can beprovided with a bellows-like expansion joint that will help reducetensile or compressive loading between the tube sheets and the tubes andthe tubesheets and the shell arising from the existence of a temperaturegradient between the tubes and shell which would tend to cause anunequal expansion or contraction therebetween.

U.S. Pat. No. 5,699,671 discloses a shell and tube heat exchanger thatcan be used as a condenser reboiler within a double column arrangementof an air separation plant such as has been described above. The type ofheat exchanger shown in this patent is known as a downflow heatexchanger because the condensing liquid flows in a downward direction ofthe tubes. In this patent a reservoir is provided for collecting theoxygen-rich liquid. The tubes penetrate the tube sheet and extend intothe reservoir to receive the oxygen-rich liquid. A central conduitextends downwardly, into the reservoir and is in registry with the inletfor the nitrogen-rich vapor situated within the tubesheet to feed thenitrogen-rich vapor into the shell. The oxygen-rich liquid flowsdownwardly through the tubes and partially vaporizes. The liquid that isnot vaporized is collected as a liquid column bottoms within the lowerpressure column and the resulting vapor provides boilup within the lowerpressure column. It is to be noted that the internal surface of each ofthe tubes can be provided with a thin metallic film coating having ahigh porosity and a large interstitial surface area to increase thesurface area for boiling.

In any application of a shell and tube heat exchanger, it is importantthat the heat transfer area per unit volume available for indirectlyexchanging heat between the vapor and the liquid be as large as possibleso that the heat exchanger is as compact as possible. In air separation,this heat transfer area will also have a major effect on the costs ofoperation of the plant as well as the profitability of the sale of theseparated components of the plant, for instance, liquid oxygen producedin the lower pressure column. The reason for this relates to theoperation of the higher and lower pressure columns. At the loweroperational pressure of the lower pressure column, the oxygen-richliquid is sufficiently cold enough to condense the higher pressure,nitrogen-rich vapor produced in the higher pressure column or in otherwords create a sufficient temperature difference across the tubes tocondense the higher pressure, nitrogen-rich vapor. As this temperaturedifference is decreased, the saturation temperature of the higherpressure, nitrogen-rich vapor will be lower. As result, the degree towhich the air needs be compressed will also be lower. Since a majorexpense in operating an air separation plant is its electrical powercosts incurred in motors used to drive compressors that compress theair, it is desirable that the plant be operated at pressure that is aslow as possible.

The heat transfer area will have a direct effect on the temperaturedifference; namely, the higher the heat transfer area provided by thetubes, the lower the temperature difference. Therefore, in a shell andtube heat exchanger, particularly, for use in an air separation plant,it is desirable to have as many tubes as possible within the shell tomaximize the heat transfer area through which heat transfer can occur.However, the problem with simply increasing the number of tubes in thesame volume is that pressure drop within the heat exchanger will alsoincrease. As can be appreciated, as the number of tubes is increased,the space between the tubes decreases resulting in the higher pressuredrop for the vapor as its proceeds in the outward, radial direction.However, as the pressure drop increases, the advantage of providing moretubes to increase the heat transfer area diminishes given that the vaporto be condensed nevertheless has to be compressed to a sufficientpressure to compensate for the increased pressure drop.

As will be discussed, the present invention provides a shell and tubeheat exchanger and a distillation column arrangement for an airseparation plant in which, among other advantages, the tubes are in anarrangement that will decrease pressure drop and allow for more tubes tobe used to increase heat transfer area.

SUMMARY OF THE INVENTION

The present invention provides a shell and tube heat exchanger thatcomprises two opposed tube sheets, a cylindrical shell connecting thetwo opposed tube sheets, a central vapor inlet and a central liquidoutlet. The central vapor inlet is centrally positioned with respect toa central axis of the shell to introduce a vapor into the shell. Thecentral liquid outlet is centrally positioned with respect to thecentral axis of the shell for discharging condensate produced bycondensing the vapor. In a specific embodiment of the present invention,the central vapor inlet can be located in one of the two opposed tubesheets and the central liquid outlet can be located in the other of thetwo opposed tube sheets.

Tubes connect the two opposed tube sheets for indirectly exchanging heatbetween a liquid flowing within the tubes and the vapor, thereby,condensing the vapor and producing the condensate within the cylindricalshell. The condensation of the vapor, at least in part, induces a flowof the vapor in an outward, radial direction toward the shell. The tubesare arranged in an inner array of the tubes spaced apart from oneanother and surrounding the central vapor inlet and the central liquidoutlet and an outer array of the tubes surrounding the inner array ofthe tubes and having a greater number of tubes than the inner array ofthe tubes. The inner array of the tubes and the outer array of the tubesare spaced apart from one another to present areas between the tubes.The areas of the inner array of the tubes have an average of the areasgreater than that of the areas of the tubes of the outer array of thetubes situated directly adjacent the inner array of the tubes to lowerpressure drop of the flow of the vapor in the outward, radial direction.

Since there will be an average open area between tubes greater than thatof the tubes of the outer array situated directly adjacent the innerarray of tubes, the velocity of the gas will be reduced to a level thatis less than that which would otherwise have been obtained had the innerand outer array presented the same average area between tubes. Thisreduction in velocity will reduce pressure drop of the vapor as it flowsin the outward, radial direction towards the shell. Since there will besome condensation of the vapor due to the heat transfer provided at theinner array of tubes, the flow and therefore, the velocity of the vaporwill be reduced after passage of the vapor through the inner array oftubes to also reduce pressure drop within the flow of the vapor.Consequently, it is possible to stack more tubes within the heatexchanger than would have been possible if all of the tubes were set atan equal spacing.

The present invention also provides a distillation column arrangementfor an air separation plant that comprises, a higher pressuredistillation column, a lower pressure distillation column and acondenser reboiler. The higher pressure distillation column isconfigured to separate nitrogen from the air and thereby to produce anitrogen-rich vapor column overhead and a crude liquid oxygen columnbottoms. The lower pressure distillation column is configured to furtherrefine the crude liquid oxygen and thereby to produce an oxygen-richliquid and a lower pressure nitrogen-rich vapor column overhead. Thecondenser reboiler partially vaporizes an oxygen-rich liquid produced inthe lower pressure column and condenses at least part of thenitrogen-rich vapor column overhead produced in the higher pressurecolumn. A means is provided for introducing the oxygen-rich liquidwithin tubes of the condenser reboiler. The condenser reboiler has thefeatures of the shell and tube heat exchanger discussed above. In thisregard, the condenser reboiler is provided with the two opposed tubesheets, a cylindrical shell, a central vapor inlet, a central liquidoutlet. The central vapor inlet is centrally positioned with respect toa central axis of the shell and connected to an inlet conduitcommunicating between the central vapor inlet and the higher pressurecolumn to receive a nitrogen-rich vapor stream composed of thenitrogen-rich vapor column overhead and thereby introduce thenitrogen-rich vapor stream into the shell. The central liquid outlet iscentrally positioned with respect to the central axis of the shell andconnected to a piping network having a first conduit connected to thehigher pressure column for introducing a reflux stream composed of thepart of the nitrogen-rich liquid into the higher pressure column and asecond conduit connected to the lower pressure column for introducinganother reflux stream composed of another part of the nitrogen-richliquid into the lower pressure column. In a specific embodiment, thecentral vapor inlet can be located in one of the two opposed tube sheetsand the central liquid outlet can be located in the other of the twoopposed tube sheets.

The tubes of the condenser reboiler connect the two opposed tube sheetsfor indirectly exchanging heat between the oxygen-rich liquid flowingwithin the tubes and the nitrogen-rich vapor, thereby condensing thenitrogen-rich vapor and producing the nitrogen-rich liquid within thecylindrical shell, at least partially vaporizing the oxygen-rich liquidwithin the tubes and, at least in part, inducing a flow of thenitrogen-rich vapor in an outward, radial direction toward the shell asa result of the condensation of the nitrogen-rich vapor. The tubes inthe inner and outer array of the tubes are arranged in the same manneras the shell and tube heat exchanger discussed above to lower pressuredrop of the flow of the nitrogen-rich vapor in the outward, radialdirection.

The oxygen-rich liquid can be composed of the oxygen-rich liquid columnbottoms produced in the lower pressure column. The oxygen-rich liquidcirculation means can comprise the inner array of the tubes and theouter array of the tubes open at opposite ends thereof and with thecondenser reboiler submerged within the oxygen-rich liquid columnbottoms. The oxygen-rich liquid flows within the inner array of thetubes and the outer array of the tubes through a thermosiphon effect.

In either the shell and tube heat exchanger or the condenser reboiler,at least one support can be located within the shell to support theinner array of the tubes in an intermediate location of the inner arrayof the tubes between the tube sheets to inhibit vibration within theinner array of the tubes. The at least one support can include a platehaving openings through which the inner array of the tubes pass and arethereby supported. The plate is supported within the shell and tube heatexchanger at the intermediate location and opposite to the central vaporinlet so that the plate also acts as a baffle to also help in inducingthe outward, radial flow of the vapor. The plate can be provided with astar-like outer periphery having indentations located opposite toinnermost tubes of the outer array of the tubes. This, as will bediscussed, will help in the assembly of the heat exchanger. Further, theplate can be supported by a set of supports connecting the plate to theone of the two tube sheets. A cylindrical member extends from the bottomof the plate towards the other of the two tube sheets to inhibit flow ofthe vapor around the plate in a direction taken from the one of the twotube sheets to the other of the two tube sheets.

Again, in either the shell and tube heat exchanger or the condenserreboiler, the inner array of the tubes can be arranged in a circularpattern having an equal spacing between the tubes. Alternatively, theinner array of the tubes can be arranged in a hexagonal pattern havingan equal spacing between the tubes. Further, in either the circular orhexagonal pattern of tubes, the outer array of the tubes is arranged ina repeating hexagonal pattern.

BRIEF DESCRIPTION OF THE DRAWINGS

While the specification concludes with claims distinctly pointing outthe subject matter that Applicants regard as their invention, it isbelieved that the invention will be better understood when taken inconnection with the drawings in which:

FIG. 1 is a fragmentary elevational, sectional view of a distillationcolumn arrangement in accordance with the present invention for an airseparation plant;

FIG. 2 is an elevational sectional view of a condenser reboiler, also inaccordance with the present invention, used in the distillation columnarrangement of FIG. 1 with tubes removed to illustrate internal featuresof the condenser reboiler;

FIG. 3 is a plan sectional view of a tube arrangement of the condenserreboiler shown in FIG. 1;

FIG. 4 is a plan view of an alternative embodiment of an arrangement oftubes in the condenser reboiler shown in FIG. 1;

FIG. 5 is plan view of yet a further, alternative embodiment of thearrangement of tubes in the condenser reboiler shown in FIG. 1; and

FIG. 6 is a top plan view of a baffle plate used within the condenserreboiler shown in FIG. 1.

DETAILED DESCRIPTION

With reference to FIG. 1, a distillation column arrangement 1 for an airseparation plant in accordance with the present invention is shown.Distillation column arrangement 1 has higher and lower pressuredistillation columns 2 and 3 and two condenser reboilers 4A and 4B ofthe same design linking the higher and lower pressure distillationcolumns 2 and 3 in a heat transfer relationship. The distillation columnarrangement 1 is specifically designed to conduct a distillation processin connection with a cycle known as the Linde double column cycle thathas been discussed in some detail above.

It is understood, however, that the distillation column arrangement 1 isbut one application of the present invention which has more generalapplicability to any heat exchanger of shell and tube design and in anyapplication thereof. As such, although for exemplary purposes thepresent invention is discussed below with respect to condenserreboilers, the invention and such discussion would have application toany shell and tube heat exchanger used in condensing a vapor throughindirect heat exchange with a fluid.

Distillation column arrangement 1, as well known in the art, is used inthe separation of nitrogen from oxygen to produce nitrogen and oxygenenriched products. Although not illustrated, as also well known, in anair separation plant, incoming air is compressed, purified and cooled toa temperature suitable for its rectification. The purified and cooledair is then introduced into the higher pressure distillation column 2where an ascending vapor phase is contacted with the descending liquidphase by known mass transfer contacting elements, generally indicated byreference number 10, which can be structured packing, random packing orsieve trays or a combination of packing and trays. The ascending vaporphase of the air becomes evermore rich in nitrogen as it ascends and adescending liquid phase becomes evermore rich in oxygen. As a result, abottoms liquid known as crude liquid oxygen or kettle liquid collects inthe bottom of the higher pressure column 2 and a nitrogen-rich vaporcollects in a top portion 12 thereof.

A stream of the kettle liquid that collects within the higher pressurecolumn 2 is introduced into the lower pressure column 3 for furtherrefinement. Again, ascending vapor and liquid phases are contactedwithin mass transfer contacting elements such as generally indicated byreference number 14. The liquid phase becomes evermore rich in oxygen asit descends within the lower pressure column 3 to form an oxygen-richliquid column bottoms 16. As is also known in the art, as the liquiddescends, the concentration of the argon within the liquid phase willincrease. Although not illustrated, an argon and oxygen containing vaporstream could be removed from the lower pressure column 3 and thenfurther refined in an argon column to produce an argon-rich product.Further, although not illustrated, a stream of the oxygen-rich liquidcolumn bottoms 16 could be taken as a product directly or vaporizedwithin the main heat exchanger to help cool the air or pumped and thenvaporized within a heat exchanger against a boosted pressure stream ofair to produce a product at pressure. The resulting liquid air couldalso be introduced into the lower pressure column 3 and/or the higherpressure column 2. As with the higher pressure column 2, the vapor phasewithin the lower pressure distillation column 3 will become evermorerich in nitrogen as it ascends.

The descending liquid phase in each of the higher and lower pressuredistillation columns 2 and 3 is initiated by refluxing the columns witha nitrogen-rich liquid produced by condensing the nitrogen-rich vaporcollected in the top portion 12 of higher pressure column 2 throughindirect heat exchange with the oxygen-rich liquid column bottoms 16 ofthe lower pressure column 3. All of such vapor need not, however, becondensed in that some of it could be removed as a high pressureproduct. This indirect heat exchange is carried out within condenserreboilers 4A and 4B. Although two of such heat exchangers are shown, aswould be known to those skilled in the art, there could be only one ormore than two of such heat exchangers in a specific application of thepresent invention. A stream of the nitrogen-rich vapor “A” is introducedinto an inlet conduit 18 that branches out to condenser reboiler 4A and4B through inlet branches 20 and 22. As will be discussed, thenitrogen-rich vapor indirectly exchanges heat with the oxygen-richliquid column bottoms 16 ascending in tubes thereof to partiallyvaporize the liquid and to fully condense the vapor. The liquid ascendswithin the tubes by the thermosiphon effect, discussed above. Thevaporization of the oxygen-rich liquid column bottoms 16 initiatesformation of the ascending vapor phase within lower pressuredistillation column 3 as shown by arrowheads “B”. Although notillustrated, liquid that is not vaporized is returned to the bottom ofthe lower pressure distillation column 3.

The resulting condensate that consists of nitrogen-rich liquid isdischarged from the condenser reboilers 4A and 4B by a piping network 24that includes branches 26 and 28 connected to the condenser reboilers 4Aand 4B to collect the nitrogen-rich liquid shown by arrowhead “C”. Afirst conduit 30 is connected to the higher pressure column 2 forintroducing a reflux stream “D” composed of the part of thenitrogen-rich liquid into the higher pressure column and a secondconduit 32 is connected to the lower pressure column 3 for introducinganother reflux stream “E” composed of another part of the nitrogen-richliquid into the lower pressure column. Preferably, a liquid distributor34 is provided within the top portion 12 of the higher pressure column 2to collect the reflux and distribute it to the underlying mass transfercontacting elements 10. Although not illustrated, a similar arrangementwould be used in connection with the introduction of liquid refluxstream “E” into the top of the lower pressure column 3.

With reference to FIG. 2, condenser reboiler 4A is illustrated. The samedesign would be used in condenser reboiler 4B. Condenser reboiler 4A isa shell and tube heat exchanger that is provided with two opposed tubesheets 36 and 38. A cylindrical shell 40 connects the tube sheets 36 and38. A bellows-like expansion joint 42 can be provided for purposesmentioned above, namely, differential expansion. Tube sheet 36 isprovided with a central vapor inlet 44 to allow the nitrogen-rich vapor“A” to enter the shell 40. An inlet pipe 46 can be connected to the tubesheet 36 in registry with the central vapor inlet 44 for connection ofthe central vapor inlet 44. Inlet pipe 46 is in turn connected to branch20 of the inlet conduit 18. The same provision can be made with respectto condenser reboiler 4B that also has an inlet pipe 46 connected tobranch 22 of the inlet conduit 18. A central liquid outlet 48 isprovided in the tube sheet 38 for discharging the condensate produced bycondensing the nitrogen-rich vapor and thereby forming the nitrogen-richliquid “C”. Outlet pipe 50 can be connected to the tube sheet 38 in bothcondenser reboilers 4A and 4B for connection to branches 26 and 28 ofthe piping network 24. It is to be noted that as well known in the art,other possible configurations for inlets and outlets are possible.However, in the illustrated embodiment, the central vapor inlet 44 andthe central liquid outlet 48 are centrally positioned with respect tothe central axis 2-2 of the shell 40 which is cylindrical.

The tubesheets 36 and 38 are connected by tubes 52 which are all of thesame design and diameter. It is to be noted that all of the tubes 52could be provided with an outer fluted surface and the interior of thetubes could be provided with an enhanced boiling surface such as beendiscussed above. The incoming nitrogen-rich vapor “A” will be condensedthrough indirect heat exchange with the oxygen-rich liquid columnbottoms flowing upwardly through the tubes 52 due to the thermosiphoneffect. Since the nitrogen-rich vapor “A” centrally enters the shell 40,through the central vapor inlet 44 and then flows in an outward, radialdirection where the nitrogen-rich vapor “A” is successively condensed, apressure gradient will be created in the flow of the nitrogen-rich vapordue to such condensation. The result of this gradient is that the flowof nitrogen-rich vapor “A” will be displaced from an axial flow, withrespect to shell 40, to a flow in an outward, radial direction as shownby arrowheads “A′” and “A″”. As will be discussed, a baffle plate 66 canalso be provided that will also have an effect of urging the incomingflow in the outward radial direction “A″”.

As has been discussed above, it is desirable to maximize the surfacearea provided by the tubes 52 for the indirect heat exchange. However,as the number of tubes 52 increases, the pressure drop within thecondenser reboiler 4A with respect to the nitrogen-rich vapor will alsoincrease. The reason for this is that as the number of tubes 52increases, there will be less area between the tubes for thenitrogen-rich vapor to flow and therefore, the velocity of thenitrogen-rich vapor between tubes 52, on average, will increase to inturn increase the pressure drop. This would be true in any shell andtube heat exchanger and in any application thereof. However, in case ofan air separation plant, the end effect would be increased compressionrequirements for the incoming air to the plant to overcome the increasedpressure drop that would negate the advantage of having the increasedsurface area for the indirect heat exchange.

With reference to FIG. 3, the present invention allows for a lowpressure drop operation while at the same time an increased surface areafor the indirect heat exchange by providing an inner array of tubes 54that can be arranged in a circle as shown by the dashed line 56 and anouter array of tubes 58 surrounding the inner array of tubes 54 with acloser spacing than the inner array of tubes 54 and with a greaternumber of tubes 54 than in the inner array. Specifically, the areabetween tubes 54 available for the flow of the nitrogen-rich vapor “A”in the outward, radial direction “A″” is given by a product of the space“S₁” between tubes 54 and the height of the tubes “H” divided by two. Itis understood that since baffle plate 66 is situated at half of theheight “H”, then the relevant height is “H” divided by two. However, ifa baffle plate were situated at a different level, the relevantdimension would change. Also, if a balffle plate were not present, thenof course the relevant area dimension would be equal to “H”, shown inFIG. 2. The area between tubes 58 of the outer array is given by aproduct of the space “S₂” between the tubes 58 and the height “H/2”. Asis apparent, the space “S₁” is greater than “S₂” and therefore, the areaavailable for flow of the nitrogen-rich vapor “A” between the tubes 52of the inner array is greater than that between the tubes 58. As aresult, the velocity of the nitrogen-rich vapor “A” as it flows throughthe inner array of tubes 54 is less than it would otherwise have beenthe case had all of the tubes 52 been arranged with the spacing of theouter array of tubes 58. This results in a reduced pressure drop for theflow of the nitrogen-rich vapor at least between the inner array oftubes 52. At the same time, the nitrogen-rich vapor is also beingcondensed through indirect heat exchange with the oxygen-rich liquid 16that provides a pressure gradient which in turn causes nitrogen-richvapor flow between the tubes 58 of the outer array that are locateddirectly adjacent the inner array of tubes 54. As the nitrogen-richvapor flows in the outward, radial direction, pressure drop will be lessdue to the decrease in the nitrogen mass flow. Consequently, the outerarray of tubes 58 can be packed more closely to provide an enhancedsurface area for the heat exchange between the nitrogen-rich vapor andthe oxygen-rich liquid column bottoms 16 to reduce the averagetemperature difference and therefore, the required pressure of thenitrogen-rich vapor and consequently, the degree to which the incomingair to the air separation plant has to be compressed.

With reference to FIG. 4, an inner array of tubes 54′ is provided thatis arranged in a hexagonal array shown by the shown by the dashed line60. Additionally, the outer array of tubes 58′ is arranged in arepeating hexagonal array shown by the dashed line 62. Although lessapparent in FIG. 3, the outer array of tubes 58 shown therein aresituated in such an array. The space “S₁” between the inner array oftubes 54′ is greater than the space “S₂′” of the outer array of tubes58′ to lower pressure drop in the same manner as has been discussed withthe tube arrangements shown in FIG. 3.

Although regular spacing for the inner array of tubes 54 and 54′ isillustrated in FIGS. 3 and 4, all that is required is that the averagearea between the tubes of the inner array be less than that of the outerarray of tubes that are located directly adjacent the inner array. Thisis illustrated in FIG. 5 in which the inner array of tube 54″ arearranged in a hexagonal pattern 64 as shown by the dashed line. Theinner array of tubes 54″ is formed by removing two tubes at oppositeends of the hexagonal pattern 60 shown in FIG. 4. As a result two pairsof tubes 54″ are separated by a space “S₃” and the remainder of alltubes are separated by a space “S₂′” that is less than the space “S₃”.If the average of the spaces and therefore the areas presented betweentubes 54″ and the adjacent row of tubes 58″ is compared, then suchaverage area of the inner row of tubes 54″ will be less than the averagearea of the outer row of tubes 58″. As used herein and in the claims,such “average of the areas” means an arithmetic average in which the sumof all of the areas between tubes is divided by the number of areas.This lower average area of the inner array of tubes 54″ than the nextadjacent row of tubes 58″ of the outer array will result in a decreasein pressure drop. In fact, referring to FIG. 3, if one such tube 54 ofthe inner array were removed, there would be a positive effect inreducing pressure drop because such removal would result in a decreasein velocity of the nitrogen-rich vapor. However, it is preferred thatthe spacing between tubes be regular, at least for the inner array oftubes so that the nitrogen rich vapor is distributed evenly in theoutward, radial direction. Having said this, the same is not true forthe outer array of tubes, for instance tubes 58 of FIG. 3. As the tubeslie further out from the geometric circular center of the shell, thesuperficial velocity of the nitrogen-rich vapor will decrease andtherefore outer tubes can be spaced closer than inner tubes to result ina further increase the surface area available for the indirect heattransfer.

Therefore, in accordance with the present invention, the average areafor the inner array of tubes, for instance 54, will always be less thanthat of the outer array of tubes, for instance 58, that are locateddirectly adjacent the inner array. The average area of the inner arrayis not always less than that of succeeding tubes of the outer array thatare not located directly adjacent the inner array. For clarity of thisconcept, tubes 54 of the inner array shown by the dashed line 56 and twotubes of the outer array has been labeled as tubes 58 a for purposes ofshowing the tubes 58 that are located adjacent the inner array of tubeswhich must present a greater average area than the inner array of tubes.

Rather than the hexagonal pattern shown for the outer array of tubes 58of FIG. 3, other arrangements could be used. For instance, circulararrangements could be used and with the number of tubes increasing ineach successive circular row of tubes as viewed in the outward radialdirection of flow. The hexagonal array is, however, preferred for theouter array of tubes. Although such circular arrays are possible, thehexagonal array gives a tighter spacing than a circular array and ahigher efficiency.

In the practice of the present invention, although as mentioned above,positive results can be obtained by simply removing a tube from theinner array, more predicable results can be obtained. In this regard, ina practical application of the present invention, the velocity wouldfirst be computed in the spacings “S₁” provided in the inner array oftubes 54. This would be done by dividing the mass flow by the product ofthe minimum flow area between adjacent tubes and the fluid bulk density.From this velocity, the frequency of shedding of vortices from the backof the tubes is computed as described in Heat Exchanger Design Handbook,“Flow induced vibration,” Ch. 4.6.1-4.6.6, Hemisphere PublishingCorporation (1987) and compared with the natural frequency of the tubesto make certain that the computed frequency is not at the naturalfrequency and in fact is preferably below 80 percent or above 120percent of such frequency. This frequency is computed by the followingformula.

$f_{n} = {\frac{\beta}{2\; \pi \; A}\left( \frac{EI}{M} \right)^{0.5}}$

(2)

-   -   β: Dimensionless geometry number    -   M_(o): Unit mass (kg/m)    -   A: Unit area (m²)    -   E: Young's Modulus (N/m²)    -   I: Momentum (m⁴)

Next the velocity for the adjacent row of tubes in the outer array,namely tubes 58 situated directly adjacent tubes 54, is calculated. Thisis done by dividing the mass flow by the product of the minimum flowarea and the bulk fluid density. The pressure drop can then becalculated from the first row of tubes 54 to succeeding rows using wellknown pressure drop correlations for flow across a tube bank such asdisclosed in the Heat Exchanger Design Handbook, “Banks of Plain andFinned Tubes,”, Chapters 2.2.4-7 to 11. A spacing is then chosen for theinner array of tubes 54 that will lower pressure drop from the center tothe periphery.

With reference again to FIG. 2, the incoming nitrogen-rich vapor “A” isalso deflected in the outward, radial direction by means of a baffleplate 66. Baffle plate 66 is connected to the tubesheet 36 by means of aset of supports 68. Extending from the underside of baffle plate 66 is acylindrical member 70 that helps prevent a flow of the nitrogen richvapor “A” directly to the central liquid outlet 48 and that acts to trapcomponents of the air, for instance, neon and helium, that would not becondensed within the condenser reboiler 4A. A tube 72 can be provided todischarge such incondensable substances from the lower pressure column 3through a tube 73, shown in FIG. 1, that connect to the tube(s) 72 andpenetrates the shell of the lower pressure column 3. Baffle plate 66 isset at half of the length of the tubes 54 and 58. This spacing, however,may need to be varied to ensure that vapor velocity exists at theoutermost tube 58 located adjacent the shell 40 to prevent the build upof condensable substances in the vapor.

With reference to FIG. 6, baffle plate 66 is provided with a series ofopenings 73 through which the inner array of tubes 54 pass. While theseopenings 73 are in a circular pattern, this may be varied in case ofother tube arrangements. For instance, in case of the tube arrangementshown in FIG. 4, the openings 73 would have a hexagonal pattern to matchthat of the inner array of tubes 54′. The baffle plate 66 thereby alsoserves as a central support for the inner array of tubes 54 which canvibrate due to the shedding of vortices at a rate matching the naturalfrequency of the tube, or due to turbulent buffeting resulting from highvelocity in the gap between tubes. In this regard, depending upon thelength of the tubes 52 used within a shell tube heat exchanger, such asthe illustrated condenser reboiler 4A, more than one such centralsupport for the tubes could be provided. Additionally, the outerperiphery of the baffle plate 66 is of star-like configuration that isprovided by indentations 74 that abut the row of the outer array oftubes 58 situated directly adjacent the inner array of tubes 54.Preferably, condenser reboiler is assembled in a horizontal orientationand such indentations 74 provide support for the adjacent row of tubes58 to help in the assembly process.

It is understood that baffle plate 66, while required in the condenserreboiler specifically illustrated in the Figures, is not required in allcases. For example, if a shell and tube heat exchanger were fabricatedin accordance with the present invention that has a lesser height “H”than that illustrated, then a baffle plate and intermediate support ofthe inner array of tubes 54 might not be required.

As mentioned previously, the above discussion would have applicabilityto the design of any shell and tube heat exchanger. It would also haveapplication to any shell and tube heat exchanger used in connection witha double distillation column for an air separation plant. In thisregard, although the condenser-reboiler was illustrated as athermosiphon type of heat exchanger, a condenser reboiler in accordancewith the present invention could also be constructed as a down flowtype. In such case, a liquid reservoir would receive oxygen-enrichedliquid from the overlying mass transfer contacting element which couldbe structured packing as shown by reference number 14. The collectedliquid would then be distributed to the tubes which would flow in adownward direction where it would be partially vaporized through theindirect heat exchange with the nitrogen-rich vapor. The liquid phasewould collect as the oxygen-rich liquid column bottoms and the vaporphase would provide boilup in the low pressure column 3.

While the present invention has been described with reference topreferred embodiments, as will be understood by those skilled in theart, numerous additions and omission can be made without departing fromthe spirit and scope of the present invention as set forth in theappended claims.

We claim:
 1. A shell and tube heat exchanger comprising: two opposedtube sheets; a cylindrical shell connecting the two opposed tube sheets;a central vapor inlet, centrally positioned with respect to a centralaxis of the shell, to introduce a vapor into the shell; a central liquidoutlet, centrally positioned with respect to the central axis of theshell, for discharging condensate produced by condensing the vapor;tubes connecting the two opposed tube sheets for indirectly exchangingheat between a liquid flowing within the tubes and the vapor, thereby,condensing the vapor and producing the condensate within the cylindricalshell and, at least in part, inducing a flow of the vapor in an outward,radial direction toward the shell as a result of the condensation of thevapor; the tubes arranged in an inner array of the tubes spaced apartfrom one another and surrounding the central vapor inlet and the centralliquid outlet and an outer array of the tubes surrounding the innerarray of the tubes and having a greater number of tubes than the innerarray of the tubes; and the inner array of the tubes and the outer arrayof the tubes spaced apart from one another to present areas between thetubes, the areas of the inner array of the tubes having an average ofthe areas greater than that of the areas of the tubes of the outer arrayof the tubes situated directly adjacent the inner array of the tubes tolower pressure drop of the flow of the vapor in the outward, radialdirection.
 2. The shell and tube heat exchanger of claim 1, wherein: thecentral vapor inlet is located in one of the two opposed tube sheets;and the central liquid outlet is located in the other of the two opposedtube sheets;
 3. The shell and tube heat exchanger of claim 2, wherein atleast one support located within the shell supports the inner array ofthe tubes in an intermediate location of the inner array of the tubesbetween the tube sheets to inhibit vibration within the inner array ofthe tubes.
 4. The shell and tube heat exchanger of claim 3, wherein theat least one support includes: a plate having openings through which theinner array of the tubes pass and are thereby supported; and the platesupported within the shell and tube heat exchanger at the intermediatelocation and opposite to the central vapor inlet so that the plate alsoacts as a baffle to also help in inducing the outward, radial flow ofthe vapor.
 5. The shell and tube heat exchanger of claim 3, wherein theplate has a star-like outer periphery having indentations locatedopposite to innermost tubes of the outer array of the tubes.
 6. Theshell and tube heat exchanger of claim 4, wherein: the plate issupported by a set of supports connecting the plate to the one of thetwo tube sheets; and a cylindrical member extends from the bottom of theplate towards the other of the two tube sheets to inhibit flow of thevapor around the plate in a direction taken from the one of the two tubesheets to the other of the two tube sheets.
 7. The shell and tube heatexchanger of claim 1 or claim 6, wherein the inner array of the tubes isarranged in a circular pattern having an equal spacing between thetubes.
 8. The shell and tube heat exchanger of claim 1 or claim 6,wherein the inner array of the tubes is arranged in a hexagonal patternhaving an equal spacing between the tubes.
 9. The shell and tube heatexchanger of claim 7, wherein the outer array of the tubes is arrangedin a repeating hexagonal pattern.
 10. The shell and tube heat exchangerof claim 8, wherein the outer array of the tubes is arranged in arepeating hexagonal pattern.
 11. A distillation column arrangement foran air separation plant comprising: a higher pressure distillationcolumn configured to separate nitrogen from the air and thereby toproduce a nitrogen-rich vapor column overhead and a crude liquid oxygencolumn bottoms; a lower pressure distillation column configured tofurther refine the crude liquid oxygen and thereby to produce anoxygen-rich liquid and a lower pressure nitrogen-rich vapor columnoverhead; a condenser reboiler for condensing at least part thenitrogen-rich vapor column overhead produced in the higher pressurecolumn and for partially vaporizing an oxygen-rich liquid produced inthe lower pressure column; means for introducing the oxygen-rich liquidwithin tubes of the condenser reboiler; and the condenser reboilercomprising; two opposed tube sheets; a cylindrical shell connecting thetwo opposed tube sheets and located within a bottom region of the lowerpressure column; a central vapor inlet centrally positioned with respectto a central axis of the shell and connected to an inlet conduitcommunicating between the central vapor inlet and the higher pressurecolumn to receive a nitrogen-rich vapor stream composed of thenitrogen-rich vapor column overhead and thereby introduce thenitrogen-rich vapor stream into the shell; a central liquid outletcentrally positioned with respect to the central axis of the shell andconnected to a piping network having a first conduit connected to thehigher pressure column for introducing a reflux stream composed of thepart of the nitrogen-rich liquid into the higher pressure column and asecond conduit connected to the lower pressure column for introducinganother reflux stream composed of another part of the nitrogen-richliquid into the lower pressure column; the tubes of the condenserreboiler connecting the two opposed tube sheets for indirectlyexchanging heat between the oxygen-rich liquid flowing within the tubesand the nitrogen-rich vapor, thereby condensing the nitrogen-rich vaporand producing the nitrogen-rich liquid within the cylindrical shell, atleast partially vaporizing the oxygen-rich liquid within the tubes and,at least in part, inducing a flow of the nitrogen-rich vapor in anoutward, radial direction toward the shell as a result of thecondensation of the nitrogen-rich vapor; the tubes arranged in an innerarray of the tubes spaced apart from one another and surrounding thecentral vapor inlet and the central liquid outlet and an outer array ofthe tubes surrounding the inner array of the tubes and having a greaternumber of tubes than the inner array of the tubes; and the inner arrayof the tubes and the outer array of the tubes spaced apart from oneanother to present areas between the tubes, the areas of the inner arrayof the tubes having an average of the areas greater than that of theareas of the tubes of the outer array of the tubes situated directlyadjacent the inner array of the tubes to lower pressure drop of the flowof the nitrogen-rich vapor in the outward, radial direction.
 12. Thedistillation column arrangement of claim 11, wherein: the central vaporinlet is located in one of the two opposed tube sheets; and the centralliquid outlet is located in the other of the two opposed tube sheets;13. The distillation column arrangement of claim 12, wherein: theoxygen-rich liquid is composed of the oxygen-rich liquid column bottomsproduced in the lower pressure column; and the oxygen-rich liquidcirculation means comprises: the inner array of the tubes and the outerarray of the tubes open at opposite ends thereof; the condenser reboilersubmerged within the oxygen-rich liquid column bottoms; and theoxygen-rich liquid flowing within the inner array of the tubes and theouter array of the tubes through a thermosiphon effect.
 14. Thedistillation column arrangement of claim 12, wherein at least onesupport located within the shell supports the inner array of the tubesin an intermediate location of the inner array of the tubes between thetube sheets to inhibit vibration within the inner array of the tubes.15. The distillation column arrangement of claim 14, wherein the atleast one support includes: a plate having openings through which theinner array of the tubes pass and are thereby supported; and the platesupported within the shell and tube heat exchanger at the intermediatelocation and opposite to the central vapor inlet so that the plate alsoacts as a baffle to also help in inducing the outward, radial flow ofthe vapor.
 16. The distillation column arrangement of claim 13, whereinthe plate has a star-like outer periphery having indentations locatedopposite to innermost tubes of the outer array of the tubes.
 17. Thedistillation column arrangement of claim 16, wherein: the plate issupported by a set of supports connecting the plate to the one of thetwo tube sheets; and a cylindrical member extends from the bottom of theplate towards the other of the two tube sheets to inhibit flow of thevapor around the plate in a direction taken from the one of the two tubesheets to the other of the two tube sheets.
 18. The distillation columnarrangement of claim 11 or claim 17, wherein the inner array of thetubes is arranged in a circular pattern having an equal spacing betweenthe tubes.
 19. The distillation column arrangement of claim 11 or claim17, wherein the inner array of the tubes is arranged in a hexagonalpattern having an equal spacing between the tubes.
 20. The distillationcolumn arrangement of claim 18, wherein the outer array of the tubes isarranged in a repeating hexagonal pattern.
 21. The distillation columnarrangement of claim 19, wherein the outer array of the tubes isarranged in a repeating hexagonal pattern.